Constant mood swings for seemingly no reason

This has been going on for what seems like forever. Sometimes I feel perfectly fine and happy, and it seems like nothing can hurt me, then something seemingly insignificant happens. For whatever reason I just fall apart. I get extremely depressed, I feel like I don't want to talk to anybody or do anything. I often think about suicide or hurting myself. Lately it seems like these thoughts have been getting worse. I've been fantasizing a lot lately about how I would kill myself (I actually attempted it a long time ago, which nobody knows about). Then usually after one of these "episodes" I go back to being normal, and I feel like an idiot for getting so worked up over nothing. I'm so mentally fatigued and stressed all the time. I just don't know what I'm going to do sometimes.